JPK,
I would think the 12:00 hold would be quite difficult because you would have a hard time seeing when you are breaking out of the bull into the white. That is why I like a 6:00 hold as you can readily see the edge of the black and it is easier to set the black on top the bead. You adjust the impact point of the group to suit your personal preferences as the black is only an aiming point.
Bill